Monolistra pretneri, an aquatic cave isopod

Did you know that Europe is extraordinarily rich in cave fauna? Particularly the Dinaric Karst of the Western Balkans is ranked high among the global hotspots of hypogean biodiversity. Everyone knows the Olm, Proteus anguinus, a highly adapted troglobiontic amphibian species endemic to this area. But there’s an incredible number of other cave-dwellers, most of them hardly known to us people above the surface, and probably just as many are still to be discovered.

Monolistra pretneri, strikingly carrying two aquatic beetle larvae on its back

To me, one of the most fascinating and diverse elements of the European underworld are the aquatic crustaceans. You might be familiar with Niphargus, an extremely widespread genus of Gammarus-like freshwater scuds of the family Niphargidae, occuring in cave and spring habitats throughout (not only) Europe.

Much more striking is Sphaeromides virei, a comparatively large, predatory troglobiontic freshwater isopod that is, like most true troglobionts, blind and unpigmented. It is very reminescent of the marine giant isopods of the genus Bathynomus and actually belongs to the same family, Cirolanidae, even though it doesn’t grow quite as big. Another genus of freshwater isopods with marine origin, Monolistra, belongs to the family Sphaeromatidae. Related genera like Sphaeroma typically occur in coastal marine and brackish water habitats, while Monolistra is restricted to subterranean freshwater bodies and springs of the Adriatic drainage. The bodies of many of the currently described 17 species are covered with spines and tubercles. Of the species depictured here, found in a cave near Vransko Jezero in Croatia, there exists a form with spines (M. pretneri ‘spinulosa’) and a smooth-bodied morph. The individual shown belongs to the latter.